The present invention relates to a method and apparatus for evaluating validation test results for critical vehicle safety systems, such as an occupant classification system associated with a passenger airbag deployment system. In particular, the invention relates to a method and apparatus for assuring that a newly designed system for classifying an occupant of a vehicle seat has the capability of demonstrating minimum reliability, with a desired level of confidence.
In order to limit the risk of death or serious injury in the event of a crash, modern vehicles include passenger airbags, which are designed to deploy in the event that the vehicle impacts with another object in certain cases. For this purpose, various types of sensors may be used, and criteria are established for determining when a qualified impact has occurred. However, with such systems, it is known that deployment of certain airbags can itself cause serious injuries in those instances where the occupant of the vehicle seat is a small child, or an infant in a Rearwardly Facing Infant Seat (RFIS). Accordingly, regulations have been developed requiring a system for distinguishing those situations in which the seat is occupied by a small child or an RFIS (in which case, the airbag deployment should be suppressed or limited in its power), from those by an adult (in which the case the airbag deployment should be enabled).
Various types of systems have been proposed for making the distinction referred to previously, such as U.S. Pat. No. 6,438,477; U.S. Pat. No. 6,438,476; U.S. Pat. No. 6,246,936; and U.S. Pat. No. 6,012,007. In general these systems utilize sensors to measure parameters that characterize the vehicle seat occupant. For example, such sensors may generate output signals indicative of occupant weight. In a crash situation, (in which the crash sensors referred to previously output signals indicative of an impact with an object), a determination is made whether the output signals from seat occupant sensors exceed a threshold value (hereinafter sometimes referred to as a xe2x80x9csuppression/enable thresholdxe2x80x9d). If so, deployment of the passenger airbag is enabled. If not, it is assumed that the occupant of the vehicle seat is either a small child or an RFIS, and deployment of the airbag is suppressed or limited in its power.
When a new system is designed for discriminating between children (including RFIS) and small adults, it is necessary that it be tested in order to establish that the system demonstrates, with acceptable reliability at a level of confidence, that it is capable of complying with certain federal motor vehicle safety standards. That is, the system must discriminate between children and small adults, so that the passenger airbag can be suppressed or enabled in accordance with the certification requirements of the federal regulation. In order to assure, to a desirable level of confidence, that the new system satisfies the established regulatory requirements, it is necessary to conduct tests.
Since the classification for systems such as airbag deployment controls generates a xe2x80x9cyes/noxe2x80x9d type result (that is, occupants will be classified as satisfying the criteria for airbag deployment, or not) in a critical safety system, conventional test methods also look for a yes/no outcome. This requires a very large number of test properties to be tested for typical Reliability goals. That is, demonstration of the requisite reliability in such circumstances has heretofore required that large numbers of systems and/or components be tested, in order to establish that the newly designed system properly distinguishes between small adults and children (including RFIS) with sufficient consistency. Typically, the systems or components in question must be evaluated prior to volume production when the cost of prototypes is quite high. Accordingly, the need to test large numbers of systems increases development costs significantly, and imposes a substantial time requirement.
It is an object of the present invention to provide a method and apparatus for verifying that an occupant sensing system will reliably classify occupants in accordance with the requirements of the federal motor vehicle safety standards.
Another object of the invention is to provide a method and apparatus for evaluating the capability of the system to discriminate between relevant classes of occupants, e.g., children and small adults, such that the passenger airbag can be suppressed or enabled in accordance with the certification requirements of the federal regulation.
Still another object of the invention is to provide a method and apparatus which can demonstrate satisfactory reliability of the system, with a small number of test samples.
Finally, still another object of the invention is to provide a method and apparatus for demonstrating that a critical system, which generates a pass/fail type of result achieves a satisfactory level of reliability, with a desired degree of confidence, using a relatively small number of test samples.
The method and apparatus according to the invention call for testing a small number of systems (vehicles or seats which include the discrimination system that is to be tested) to demonstrate the minimum reliability of a passenger occupant classification system for controlling a passenger airbag, while retaining a statistically sound basis for inferring minimum reliability at a satisfactory confidence level. The method calls for placing xe2x80x9climiting casexe2x80x9d Anthropomorphic Test Devices (ATDs), sometimes referred to as xe2x80x9ccrash test dummiesxe2x80x9d, in a passenger seat which contains the system being tested. The limiting case ATDs are those which represent the largest occupant that is to be classified as suppressing an airbag, and the smallest occupant which is to be classified as enabling the airbag. For example, a classification causing the passenger airbag to be suppressed will occur for a child or RFIS, while a classification causing the passenger airbag to be enabled will occur for a fifth percentile female (a female which falls at the fifth percentile for size or weight). The sensor output that is used to classify the occupant is then recorded for both limiting case ATDs for each sample of the system being tested. (The sensor output may, for example, be indicative of weight, cushion pressure, a weighted summation of sensor outputs, or other scalar parameter.) Tests are then run for a series of xe2x80x9cnxe2x80x9d systems (samples), as described hereinafter.
The mean or average sensor output value, and the standard deviation are then computed for the n samples of sensor output, for each of the two limiting case ATDs. Assuming that the sensor system utilized is such that a larger occupant produces a larger value of the sensed parameter, two xe2x80x9cmarginsxe2x80x9d are then computed. The suppress limiting case (child/RFIS) margin is computed as the surplus of the suppression/enable threshold value which is to be used for discriminating between the two types of occupants beyond a value determined as the mean of the suppress limiting case measured values plus a constant multiplied by the sample standard deviation of the suppress limiting cases. The enable limiting case margin is computed as the surplus of a quantity calculated as the mean of the enable limiting cases minus a constant multiplied by the standard deviation of the enabling limiting case, beyond the suppression/enable threshold. The constant is selected from known statistical lookup tables (based on the Non-Central t-Distribution), tabulated corresponding to the number of samples (typically three to eight), the minimum reliability that is to be demonstrated, and the desired degree of confidence. It is assumed that the measurement reading for a given ATD in a vehicle is a random variable whose distribution is Normal (bell-shaped) across the vehicle population.
If both of the two xe2x80x9cmarginsxe2x80x9d calculated in the manner described above are non-negative, the minimum level of reliability at the desired confidence level is demonstrated by the system.
The technique according to the invention can also be used to evaluate the feasibility of setting a xe2x80x9csuppression/enable threshold valuexe2x80x9d for a particular sensing system, such that the system is able to reliably discriminate between two classes of seat occupants. For this purpose, assuming once again that a larger occupant produces a larger value of the sensed parameters, two xe2x80x9cboundsxe2x80x9d are computed as follows. The suppress limiting case bound is the mean of the values sensed for the suppressed limiting cases plus an appropriate constant multiplied by the sample standard deviation of the suppress limiting cases. The enable limiting case bound is then determined as the mean of the enable limiting cases minus the appropriate constant times the standard deviation of the enabling limiting cases. (The constant is selected in the manner described previously.) If the latter bound exceeds the former, then it is feasible to set a threshold while demonstrating the minimum level of reliability at the desired confidence for the system under test.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.